Known piezo injectors are typically actuated by means of piezo actuators and permit a very fast and precise dosing of the fuel quantity, and are described for example in the reference book “Diesel- and Benzindirekteinspritzung” [“Diesel and gasoline direct injection”], Prof. Dr.-Ing. Helmut Tschöke et al., Expert Verlag 2001.
The switching times of piezo injection valves, which are up to four times faster than those of known systems, permit short and variable intervals between the individual injections, for example pilot, main and post injections. Very short switching times are possible. As a result, the injected fuel quantity can be controlled and dosed very precisely. Excellent repeatability is furthermore ensured. However, since the actuating movements that can be generated by means of piezo actuators are very small and the pressure forces possibly to be overcome are very large, the opening and closing of such an injection valve takes place hydraulically, utilizing the fuel pressure, wherein the piezo actuator serves merely to switch a control valve and thereby create the pressure difference required in each case.
In a known embodiment, a high-pressure injection valve of said type has substantially the following functional units:                a piezo actuator with a stroke booster and a control valve piston,        a cylindrical valve shank with a control chamber, a servo control valve, a control plunger and a closing spring chamber, and        a valve tip with spray holes, a needle seat, a high-pressure ring chamber and a nozzle needle.        
Here, the fuel high pressure of the common rail acts, in the control chamber, on the rear end of the control plunger and, in a high-pressure annular chamber, on a pressure shoulder of the nozzle needle. As a result of the annular gaps, which result from the construction, between the control plunger and the associated receiving bore in the valve shank and also between the nozzle needle and the associated receiving bore in the valve tip, there is a constant fuel loss flow which is referred to as permanent leakage.
Said permanent leakage which has an ever greater effect with rising system pressure constitutes an ever greater problem for injection systems of the future, because it necessitates an ever more powerful high-pressure pump.